Method and apparatus for performing handover using packet data convergence protocol (PDCP) reordering in mobile communication system

ABSTRACT

Improving communication efficiency in handover re-establishing an ARQ entity in a mobile communication system is disclosed. The method includes transmitting first Packet Data Convergence Protocol (PDCP) Packet Data Units (PDUs) correctly received from the source cell, together with a special indication requiring reordering of the first PDCP PDUs, from a Radio Link Control (RLC) receiving buffer to a PDCP receiving entity when a handover command message from a source cell to a target cell is received; buffering the first PDCP PDUs in a PDCP PDU reordering buffer by the PDCP receiving entity in response to the special indication; and when a second PDCP PDU is received from the target cell through a new RLC receiving entity for the target cell, outputting third PDCP PDUs up to a PDCP PDU before a first missing PDCP PDU having a sequence number higher than that of the second PDCP PDU from the PDCP reordering buffer. In the method, a PDCP entity performs reordering, thereby improving efficiency of communication.

PRIORITY

This application is a Continuation Application of U.S. application Ser.No. 11/875,315, which was filed in the U.S. Patent and Trademark Officeon Oct. 19, 2007, and claims priority under 35 U.S.C. §119(a) to anapplication entitled “Method And Apparatus For Performing Handover WhilePDCP Reordering In Mobile Communication System” filed in the KoreanIndustrial Property Office on Oct. 19, 2006 and assigned Serial No.10-2006-0101842, the contents of each of which are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a mobile communication system, and moreparticularly to a method and an apparatus for supporting handoverbetween cells while a User Equipment (UE) performs packet reordering.

2. Description of the Related Art

A Universal Mobile Telecommunication Service (UMTS) system is a 3^(rd)Generation (3G) asynchronous mobile communication system, which useswideband Code Division Multiple Access (CDMA) and is based on GlobalSystem for Mobile Communications (GSM) and General Packet Radio Services(GPRS), which are European mobile communication systems. The UMTS systemis an example of cellular mobile communication systems, each of whichincludes a plurality of cells divided from an entire service area of thesystem and supports handover between cells in order to guaranteecontinuity of communication.

When an active UE moves from one cell (source cell) to another cell(target cell), the target cell establishes a communication path for theUE and re-establishes entities of each layer related to the establishedcommunication path. Especially, when the communication of the UEsupports Automatic Retransmission reQuest (ARQ), it may sometimes becomenecessary for the target cell to re-establish an ARQ entity for the UE.As used here, handover re-establishing an ARQ entity refers to handoverin which the source cell removes an existing ARQ being used therein andthe target cell sets a new ARQ entity.

When handover re-establishing an ARQ entity is performed in aconventional mobile communication system, a higher layer entity of theARQ entity performs accumulative retransmission. The accumulativeretransmission can reduce the complexity of the higher layer entityalthough packets already transmitted in the source cell may beretransmitted in the target cell. In a conventional UMTS mobilecommunication system, the handover re-establishing an ARQ entity occursduring Serving Radio Network Subsystem (SRNS) reallocation during whicha serving Radio Network Controller (RNC) of a UE is changed. Because theSRNS reallocation does not frequently occur, the accumulativeretransmission is performed in consideration of complexity rather thanefficiency.

Meanwhile, in the 3rd generation Partnership Project (3GPP), which is incharge of standardization of the UMTS, active discussion about Long TermEvolution (LTE) of the UMTS system as an evolved mobile communicationsystem of the UMTS system is ongoing. The LTE is technology, which istargeting commercialization thereof by the year 2010 and the realizationof high speed packet-based communication at a speed of about 100 Mbps.To this end, various schemes are being discussed, which include a schemefor reducing the number of nodes located in a communication path bysimplifying the structure of a network, and a scheme for approaching awireless protocol to a wireless channel as much as possible.

In an evolved mobile communication system such as the LTE system, an ARQentity is located in a Node B that is a lower layer entity of the RNC.Therefore, an ARQ entity is always re-established in handover among NodeBs, and thus the handover re-establishing an ARQ entity occurs much morefrequently in an evolved mobile communication system, such as the LTEsystem, than the UMTS system. Therefore, it is necessary to developtechnology for improving communication efficiency in the handoverre-establishing an ARQ entity in an evolved mobile communication system,such as the LTE system.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve theabove-mentioned problems occurring in the prior art, and the presentinvention provides a method and an apparatus for improving communicationefficiency in handover re-establishing an ARQ entity in a mobilecommunication system.

In addition, the present invention provides a method and an apparatusfor preventing loss or repeated transmission of data packets in handoverre-establishing an ARQ entity.

In addition, the present invention provides a method and an apparatusfor reordering by a higher layer entity of an ARQ entity, which canselectively retransmit packets not received in a source cell when a UEmoved to a target cell.

In accordance with an aspect of the present invention, there is provideda method for performing handover by a User Equipment (UE) in a mobilecommunication system, the method includes transmitting first Packet DataConvergence Protocol (PDCP) Packet Data Units (PDUs) correctly receivedfrom the source cell, together with a special indication requiringreordering of the first PDCP PDUs, from a Radio Link Control (RLC)receiving buffer to a PDCP receiving entity when a handover commandmessage from a source cell to a target cell is received; buffering thefirst PDCP PDUs in a PDCP PDU reordering buffer by the PDCP receivingentity in response to the special indication; and when a second PDCP PDUis received from the target cell through a new RLC receiving entity forthe target cell, outputting third PDCP PDUs up to a PDCP PDU before afirst missing PDCP PDU having a sequence number higher than that of thesecond PDCP PDU from the PDCP reordering buffer.

In accordance with another aspect of the present invention, there isprovided a UE apparatus for performing handover in a mobilecommunication system, the UE apparatus includes an existing RLCreceiving entity for receiving RLC PDUs from a source cell andassembling the RLC PDUs into PDCP PDUs before handover, and outputtingfirst PDCP PDUs correctly received from the source cell, together with aspecial indication requiring reordering of the first PDCP PDUs, when ahandover command message from a source cell to a target cell isreceived; a new RLC receiving entity for receiving one or more RLC PDUscontaining a second PDCP PDU, which has not been correctly received fromthe source cell, from the target cell after the handover, and assemblingthe received RLC PDUs into the second PDCP PDU; and a PDCP receivingentity for buffering the first PDCP PDUs in a PDCP PDU reordering bufferin response to the special indication, and outputting third PDCP PDUs upto a PDCP PDU prior to a first missing PDCP PDU having a sequence numberhigher than that of the second PDCP PDU from the PDCP reordering bufferwhen the second PDCP PDU is delivered from the new RLC receiving entity.

In accordance with another aspect of the present invention, there isprovided a method for performing handover by an Evolved Node B (ENB) ina mobile communication system, the method includes receiving by a targetENB controlling the target cell from the source ENB, first PDCP PDUs,which were not correctly received by a UE from a source ENB controllinga source cell due to handover of the UE from the source cell to a targetcell; transmitting second PDCP PDUs together with a first indicationfrom the target ENB to the UE, the second PDCP PDUs includes remainingPDCP PDUs of the first PDCP PDUs except for a last PDCP PDU from amongthe first PDCP PDUs, which is the first indication that each of thesecond PDCP PDUs is not the last PDCP PDU delivered from the source cellto the target cell; and transmitting the last PDCP PDU from the targetENB to the UE together with a second indication that the last PDCP PDUis the last PDCP PDU delivered from the source cell to the target cell.

In accordance with another aspect of the present invention, there isprovided an Evolved Node B (ENB) apparatus for performing handover in amobile communication system, the ENB apparatus includes a transmissionbuffer for storing first PDCP PDUs delivered from a source ENBcontrolling a source cell and storing PDCP PDUs delivered from an anchornode, wherein a UE has not correctly received the first PDCP PDUs fromthe source ENB due to handover of the UE from the source cell to atarget cell; and a control unit for controlling the transmission buffersuch that the transmission buffer transmits second PDCP PDUs togetherwith a first indication to the UE and the last PDCP PDU together with asecond indication to the UE, the second PDCP PDUs includes remainingPDCP PDUs of the first PDCP PDUs except for a last PDCP PDU from amongthe first PDCP PDUs, the first indication indicating that each of thesecond PDCP PDUs is not the last PDCP PDU delivered from the source cellto the target cell, the second indication indicating that the last PDCPPDU is the last PDCP PDU delivered from the source cell to the targetcell.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is an overview of a structure of an LTE mobile communicationsystem;

FIG. 2 illustrates a protocol stack of an LTE mobile communicationsystem;

FIG. 3 illustrates an operation of an RLC layer in a mobilecommunication system;

FIG. 4 is a signal flow diagram illustrating a process of handoverre-establishing an ARQ entity when a UE moves to a new cell belonging toanother ENB;

FIG. 5 is a message flow diagram illustrating an example of the entireprocess according to the present invention;

FIG. 6 is a flow diagram illustrating a process of RLC reception by a UEaccording to the present invention;

FIG. 7 is a flow diagram illustrating an operation of a PDCP receivingentity according to the present invention;

FIG. 8 illustrates an example of RLC control information according tothe present invention;

FIG. 9 is a message flow diagram illustrating an example of the entireprocess according to the present invention;

FIG. 10 illustrates a flow diagram illustrating a process of RLCreception by a UE according to the present invention;

FIG. 11 is a flow diagram illustrating an operation of a PDCP receivingentity according to the present invention; and

FIG. 12 is a block diagram illustrating structures oftransmitting/receiving entities according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. In the followingdescription, detailed description of known functions and configurationsincorporated herein will be omitted when it may make the subject matterof the present invention rather unclear. The main objective of thepresent invention is to prevent repeated transmission of data packets byusing selective retransmission when handover re-establishing an ARQentity occurs. Presently, a higher layer entity of the ARQ entityperforms proper reordering operation for received data packets, so thatselectively retransmitted packets are delivered to the higher layer inthe same order as that in which the packets have been originallytransmitted.

An LTE system will be briefly described as an illustrative example ofthe embodiments of the present invention.

Referring to FIG. 1, an Evolved UMTS Radio Access Network (E-RAN) 110 or112 has a simplified 2 node structure, which includes Evolved Node Bs(ENBs) 120, 122, 124, 126, and 128, and anchor nodes 130 and 132. A UserEquipment (UE) 101 is connected to an Internet Protocol (IP) network 114through the E-RANs 110 and 112.

ENBs 120 to 128 correspond to existing Node Bs of the UMTS system andare connected to UE 101 through a wireless channel. ENBs 120 to 128perform scheduling by collecting situation information and havefunctions related to radio resource control. For example, the ENB isequipped with a control protocol, such as a Radio Resource Control (RRC)protocol.

In order to realize a maximum of a transmission speed of 100 Mbps, theLTE uses an Orthogonal Frequency Division Multiplexing (OFDM) scheme ina 20 MHz system bandwidth as wireless access technology. Further, theLTE system uses an Adaptive Modulation and Coding (AMC) scheme, whichdetermines a modulation scheme and a channel coding rate in accordancewith the channel status of a UE.

Referring to FIG. 2, Packet Data Convergence Protocol (PDCP) layers 205and 240 perform operations, such as compression/decompression andciphering/deciphering of an IP header, thereby generating PDCP PacketData Units (PDUs). As used herein, a packet output from a particularprotocol entity is called a PDU of the protocol. Each of Radio LinkControl (RLC) layers 210 and 235 functions as an ARQ entity whichreconstructs a PDCP PDU into RLC PDUs each having a proper size andperforms an ARQ operation on the RLC PDUs. PDCP layers 205 and 240 andRLC layers 210 and 235 construct at least one PDCP entity or at leastone RLC entity, which may be constructed according to each service orflow when the communication starts, and process data packets througheach entity. As shown in FIG. 2, PDCP layers 205 and 240 are located inthe UE and the anchor node, respectively, and RLC layers 210 and 235 arelocated in the UE and the ENB, respectively.

Medium Access Layer (MAC) layers 215 and 230 are connected to aplurality of RLC entities and perform multiplexing of RLC PDUs into aMAC PDU and demultiplexing of a MAC PDU into RLC PDUs. Physical layers220 and 225 generate OFDM symbols by channel-coding and modulatinghigher layer data and transmit the generated OFDM symbols through awireless channel, or demodulate and channel-decode OFDM symbols receivedthrough a wireless channel and transfer the OFDM symbols to a higherlayer. Most of Hybrid ARQ (HARQ) operations, such as channel decoding ofreceived packets, soft combining of packets with previously-receivedpackets, CRC calculation, etc., are performed in physical layers 220 and225, and MAC layers 215 and 230 control the HARQ operations.

As described above, RLC layers 210 and 235 guarantee reliable datatransmission/reception through the ARQ process. Based on the foregoing,an entity of the RLC layer is called an ARQ entity.

Referring to FIG. 3, a transmission buffer 305 of a transmitter-side RLClayer stores PDCP PDUs 310 and 312 provided by the transmitter-side RLClayer, before transmitting the PDCP PDUs to a receiver-side RLC layer.Each of the PDCP PCUs includes a ciphered and header-compressed IPpacket in a payload and includes a PDCP sequence number sequentiallyincreasing one by one in a header. The sequence number corresponds to an“input value changing according to each packet,” which is used forciphering and deciphering of an IP packet. In most of currently knownciphering schemes, the security of ciphering is enhanced by using the“input value changing according to each packet” when a cipheringapparatus ciphers a packet. PDCP PDUs 310 and 312 are reconstructed intoRLC PDUs each having the proper size by a framing unit 315, RLC sequencenumbers increasing one by one are attached to the reconstructed RLCPDUs, and the RLC PDUs are transmitted to the receiver-side RLC layer.Then, the RLC PDUs are buffered in a retransmission buffer 320 when anAcknowledgement (ACK) signal is received from the receiver-side RLClayer.

The receiver-side RLC layer stores the received RLC PDUs in a receptionbuffer 330, detects RLC PDUs lost during the transmission by checkingeach sequence number, and makes a request for retransmission of the RLCPDUs lost during the transmission to the transmitter-side RLC layer.Hereinafter, for convenience of description, RLC PDU [x] indicates anRLC PDU having an RLC sequence number of x, and PDCP PDU [x] indicates aPDCP PDU having a PDCP sequence number of x.

An example of an ARQ operation performed by the RLC layer will bedescribed. At one time point, from among RLC PDU [7] to RLC PDU [10]transmitted from the transmitter-side RLC layer, only RLC PDU [8] andRLC PDU [9] have been received by the receiver-side RLC layer and arestored in reception buffer 330. The receiver-side RLC layer replies tothe transmitter-side RLC layer by sending a status report 340 reportingcorrect reception of RLC PDU [8] and RLC PDU [9] and failure inreceiving RLC PDU [7]. Specifically, status report 340 includes ACK[8,9], which is an ACK signal including sequence numbers of 8 and 9, andNACK [7], which is a NACK signal including a sequence number of 7. Then,the transmitter-side RLC layer retransmits RLC PDU [7], which is storedin retransmission buffer 320 and requested to be retransmitted, anddiscards the correctly transmitted RLC PDU [8] and RLC PDU [9] fromretransmission buffer 320. From among the RLC PDUs stored in receptionbuffer 330, RLC PDUs capable of constructing one complete PDCP PDU areconstructed into a PDCP PDU by a reassembly unit 335 and the constructedPDCP PDU is then transferred to the receiver-side PDCP layer.

One important characteristic of the operation performed by the RLC layeris that, when the receiver-side RLC layer delivers PDCP PDUs to thereceiver-side PDCP layer, the transmitter-side RLC layer delivers thePDCP PDUs in the same order as that in which the transmitter-side RLClayer has received the PDCP PDUs from the transmitter-side PDCP layer.This is called an “in-sequence delivery” operation. For example, eventhough RLC PDU [101] 310 can be reassembled by using RLC PDU [8] and RLCPDU [9], the receiver-side RLC layer does not reassemble RLC PDU [8] andRLC PDU [9] into RLC PDU [101] 310 and deliver the reassembled RLC PDU[101] 310 to the PDCP layer because the receiver-side RLC layer has notreceived RLC PDU [7] yet. When the receiver-side RLC layer has receivedthe retransmitted RLC PDU [7] and the reception buffer 330 does notcontain a missing RLC PDU any more, the reassembly unit 335 of thereceiver-side RLC layer constructs a PDCP PDU by reassembling the RLCPDUs stored in the reception buffer 330 and delivers the constructedPDCP PDU to the receiver-side PDCP layer.

Since the RLC layer provides a reliable transmission/reception functionand an “in-sequence delivery” function as described above, the PDCPlayer does not require a separate buffering or sequence reorderingfunction. However, when a UE performs handover to a cell belonging toanother ENB, the UE should remove an ARQ entity (i.e. an RLC entity)having been used in a previous cell and should re-establish an RLCentity to be used in a new cell. Therefore, before the handover iscompleted, the RLC layer may be unable to provide an “in-sequencedelivery” function and reliable transmission/reception function by anARQ operation.

As shown in FIG. 4, UE 405 includes a PDCP receiving entity and an RLCreceiving entity, each of a source ENB 410 controlling a source cell anda target ENB 415 controlling a target cell includes an RLC transmittingentity, and an anchor node 420 includes a PDCP transmitting entity.

Referring to FIG. 4, when RLC PDUs are transmitted between UE 405 andsource ENB 410 (step 425), source ENB 410 determines handover of UE 405to a cell of target ENB 415 (step 430). When source ENB 410 requeststarget ENB 415 to prepare itself for the handover (step 435), target ENB405 makes preparations which enable UE 405 to instantly restartcommunication after the handover to target ENB 415, for example, targetENB 405 sets an RLC entity for UE 405 (step 440), and then reports tosource ENB 410 that the preparation is completed (step 445). Then,source ENB 410 stops downlink transmission to UE 405 (step 447), andcommands UE 405 to perform handover to target ENB 415 (step 450). Here,data transmission/reception up to the time point at which the command istransmitted is based on the following assumptions.

-   -   Anchor node 420 has transmitted PCDP PDU [1] to PDCP PDU [8] to        source ENB 410.    -   Source ENB 410 has transmitted RLC PDUs corresponding to PCDP        PDU [1] to PDCP PDU [8] to UE 405. PDCP PDU [7] and PDCP PDU [8]        have not been transmitted yet.    -   From among the RLC PDUs, UE 405 has correctly received the RLC        PDUs corresponding to PCDP PDU [1], PDCP PDU [2], PDCP PDU [4],        and PDCP PDU [6] (step 449).    -   UE 405 reports to source ENB 410 through an RLC status report        that UE 405 has correctly received the RLC PDUs corresponding to        PCDP PDU [1] and PDCP PDU [2].    -   From among the sequence-ordered RLC PDUs, UE 405 assembles PCDP        PDU [1] and PDCP PDU [2] and delivers the assembled PDUs to the        PDCP receiving entity.    -   An RLC receiving entity of UE 405 stores RLC PDUs corresponding        to PCDP PDU [4] and PDCP PDU [6].

Source ENB 410 delivers PDCP PDU [3] to PDCP PDU [6], for which an ACKsignal of an RLC level has not been received yet from UE 405, and PDCPPDU [7] and PDCP PDU [8], which have not transmitted to UE 405 yet, totarget ENB 415 (step 455).

Upon receiving the handover command, UE 405 removes out-of-sequence RLCPDUs from the reception buffer and removes the existing RLC entity (theRLC entity performed communication with the source cell). Then, togetherwith the existing RLC entity, the RLC PDUs corresponding to PCDP PDU [4]and PDCP PDU [6] are removed (step 460). Thereafter, UE 405 performshandover to target ENB 415, and then constructs a new RLC entity forcommunication with target ENB 415 and transmits a handover completemessage to target Node message 415 (step 465). The handover completemessage includes sequence numbers of the correctly received PDCP PDUs.For example, because the PDCP receiving entity of UE 405 has correctlyreceived PDCP PDU [1] and PDCP PDU [2], the handover complete messageincludes information indicating that PDCP PDUs have been received up toPDCP PDU [2].

When target ENB 415 has received the handover complete message, targetENB 415 requests anchor node 420 to change the downlink data pathbecause UE 405 has performed the handover (step 467), and determines toretransmit PDCP PDU [3] to PDCP PDU [6], which the PDCP receiving entityof UE 405 has not received yet, from among the PDCP PDUs received fromsource ENB 410, by using the RLC entity newly constructed for UE 405(step 480). In response to the request from target ENB 415, anchor node420 switches the downlink data path for UE 405 from source ENB 410 totarget ENB 415, and transmits the following PDCP PDUs delivered tosource ENB 410, that is, PDCP PDU [9] to PDCP PDU [11], to target ENB415. Meanwhile, target ENB 415 transmits PDCP PDU [3] and the PDCP PDUsfollowing thereafter to UE 450 by using the newly constructed RLCentity.

As shown in FIG. 4, when target ENB 415 resumes transmission of PDCPPDUs from a PDCP PDU after the sequence-ordered PDCP PDUs in spite of apossibility that the transmission may be a repeated retransmission, thePDCP receiving entity does not need to either separately buffer thereceived PDCP PDUs or reorder the PDCP PDUs. That is, the PDCP receivingentity immediately inputs the PDCP PDUs delivered from the RLC receivingentity into a deciphering entity and a header decompression entity.

However, if the target ENB transmits only the PDCP PDUs that the UE hasnot received, the PDCP PDUs require separate buffering and reordering.The reordering refers to an operation of reordering received packetsaccording to their sequence numbers and then delivering the reorderedpackets to a next processing block while storing the out-of-sequencepackets until they become in-sequence through sequence-reordering. Here,the out-of-sequence packets imply that there is a missing packet (i.e. apacket not received yet). That is, when there is a missing packet,packets having sequence numbers higher than the sequence number of themissing packet are assumed as out-of-sequence packets. The reorderingentity temporarily stores the out-of-sequence packets either before themissing packet is received or until it is concluded that the missingpacket is completely lost. An efficient reordering operation depends onhow fast it is possible to detect a loss of the missing packet anddeliver the packets having sequence numbers higher than the sequencenumber of the lost packet to a next processing block, when it isconcluded that the missing packet has been completely lost.

According to the first embodiment of the present invention, the PDCPreceiving entity of the UE temporarily stores the out-of-sequence PDCPPDUs in a reception buffer, from among the correctly received PDCP PDUsfrom the source ENB, and when it receives a PDCP PDU from a target cell,the PDCP receiving entity determines that sequence reordering of thePDCP PDUs having sequence numbers lower than that of the received PDCPPDU has been completed. This is based on the fact that the RLC receivingentity of the UE performs the “in-sequence delivery” operation, so thatreception of a PDCP PDU having a sequence number of x by a PDCPreceiving entity implies that there is no possibility that the PDCPreceiving entity can receive another PDCP PDU having a sequence numberlower than x.

Referring to FIG. 5, before receiving the handover command from sourceENB 510, UE 505 receives RLC PDUs corresponding to PDCP PDU [1], PDCPPDU [2], PDCP PDU [4], and PDCP PDU [6] from the source ENB 510 (step520). From among the RLC PDUs, because RLC PDUs corresponding to PDCPPDU [1] and PDCP PDU [2] are already in sequence, they are assembledinto PDCP PDU [1] and PDCP PDU [2], which are then delivered to the PDCPreceiving entity first.

When UE 505 receives the handover command from source ENB 510 (step525), the UE assembles all RLC PDUs fit for assembly from among the RLCPDUs remaining in the RLC reception buffer into PDCP PDUs and thendelivers the assembled PDCP PDUs to the PDCP receiving entity (step530). At this time, PDCP PDU [4] and PDCP PDU [6] are assumed to havebeen correctly received and are delivered to the PDCP receiving entity.Also, the RLC receiving entity of UE 505 delivers to the PDCP receivingentity, together with the correctly received PDCP PDUs, a specialindication that the out-of-sequence PDCP PDUs requiresequence-reordering. From the special indication, the PDCP receivingentity recognizes that PDCP PDU [3] and PDCP PDU [5] have not beenreceived yet, and temporarily stores PDCP PDU [4] and PDCP PDU [6],which are out-of-sequence PDCP PDUs, in a reordering buffer, instead ofdelivering them to a next processing block (step 535).

After performing the handover to the target ENB 515, UE 505 transmits ahandover complete message to target ENB 515 (step 540). The handovercomplete message contains information on a PDCP PDU reception status,that is, sequence numbers of missing PDCP PDUs and sequence numbers ofreceived PDCP PDUs. In the illustrated example, the handover completemessage contains PDCP PDU reception status information that PDCP PDUs upto PDCP PDU [6] have been received, while PDCP PDU [3] and PDCP PDU [5]are missing.

After transmitting the handover complete message, UE 505 establishes anew RLC entity to be used in target ENB 515 (step 545), and waits forarrival of RLC PDUs to the new RLC receiving entity from target ENB 515.

Meanwhile, target ENB 515 receives PDCP PDUs from source ENB 510 (step527). Then, upon receiving the handover complete message from UE 505,target ENB 515 determines the PDCP PDUs to be transmitted by referringto the PDCP PDU reception status information contained in the handovercomplete message and then transmits the determined PDCP PDUs byreferring to the sequence numbers of the determined PDCP PDUs (step550). In the illustrated example, target ENB 515 transmits the PDCP PDUsin a sequence of PDCP PDU [3], PDCP PDU [5], PDCP PDU [7], and PDCP PDU[8] from among PDCP PDU [3], PDCP PDU [4], PDCP PDU [5], PDCP PDU [6],PDCP PDU [7], and PDCP PDU [8] received from source ENB 510. At thistime, PDCP PDU [4] and PDCP PDU [6] may be discarded by target ENB 515.Target ENB 515 reconstructs the PDCP PDUs into RLC PDUs in theabove-mentioned order, attaches RLC sequence numbers to the RLC PDUs,and then transmits the RLC PDUs to the RLC receiving entity of the UE.Since a sequence number of the RLC transmitting entity constructed intarget ENB 515 is initialized to 0, target ENB 515 grants a sequencenumber of 0 to the first RLC PDU corresponding to PDCP PDU [3].

Target ENB 515 and UE 505 restart normal RLC transmission/receptionthrough new RLC transmission and reception entities (step 555), and theRLC receiving entity of UE 505 assembles the reordered in-sequence RLCPDUs into PDCP PDUs and delivers the assembled PDCP PDUs to the PDCPreceiving entity (step 560). During the RLC transmission/reception,transmission/reception of a certain RLC PDU may result in a completefailure. This may happen, for example, when the RLC PDU is not correctlytransmitted with a predetermined time interval or when retransmissionhas been tried up to the maximum number of times allowed forretransmission without a successful retransmission. When reception ofthe missing RLC PDU is concluded as a failure after all, the RLCreceiving entity performs the “in-sequence delivery” operation whileignoring the existence of the missing RLC PDU. That is, from among thereordered in-sequence RLC PDUs based on an assumption that the missingRLC PDU has been received, RLC PDUs fit to be PDCP PDUs are assembledinto PDCP PDUs, which are then delivered to the PDCP receiving entity.

Since the RLC receiving entity performs the “in-sequence delivery”operation as described above, there is no possibility that another PDCPPDU having a sequence number lower than that of the PDCP PDU deliveredby the RLC receiving entity constructed for use in the target cell maybe received. Therefore, the PDCP receiving entity can determine that thePDCP PDUs are in sequence up to the PDCP PDU delivered from the RLCtransmitting entity of the target cell (step 565). For example, if thePDCP receiving entity has received PDCP PDU [5] from the new RLCreceiving entity without receiving PDCP PDU [3], the PDCP receivingentity concludes that PDCP PDU [3] having a sequence number lower thanthat of PDCP PDU [5] is completely lost and that the PDCP PDUs are insequence up to PDCP PDU [5].

The entire operation according to the first embodiment of the presentinvention can be summarized as follows:

-   -   Upon receiving a handover command, the RLC receiving entity of        the UE assembles all qualified RLC PDUs into PDCP PDUs and        delivers the PDCP PDUs to the PDCP receiving entity. At this        time, the RLC receiving entity delivers, together with the PDCP        PDUs, a special indication that the out-of-sequence PDCP PDUs        require sequence-reordering to the PDCP receiving entity.    -   Upon receiving the PDCP PDUs and the special indication, the        PDCP receiving entity checks the sequence numbers of the        received the PDCP PDUs and stores out-of-sequence PDCP PDUs,        that is, the PDCP PDUs having sequence numbers higher than the        sequence number of the missing PDCP PDU, in the reordering        buffer.    -   The PDCP receiving entity delivers a PDCP PDU reception status        to a Radio Resource Control (RRC) entity of the UE.    -   The RRC entity of the UE inserts the PDCP PDU reception status        in the handover complete message and then transmits the handover        complete message to the target ENB.    -   An RRC entity of the target ENB delivers the PDCP PDU reception        status to the RLC receiving entity constructed for the UE.    -   Based on the PDCP PDU reception status, the RLC transmitting        entity transmits PDCP PDUs according to their sequence numbers,        except for the PDCP PDUs which the UE already received from the        source ENB from among the PDCP PDUs delivered from the source        ENB.    -   After the handover complete message is transmitted, the RLC        receiving entity of the UE performs the “in-sequence delivery”        operation for the RLC PDUs received from the RLC receiving        entity of the target ENB.    -   Upon receiving a PDCP PDU from the new RLC transmitting entity,        the PDCP receiving entity of the UE assumes that the PDCP PDUs        are in sequence up to the received PDCP PDU, and delivers all        PDCP PDUs having sequence numbers lower than a sequence number        of a first missing PDCP PDU, which is higher than a sequence        number of the PDCP PDU received from the new RLC transmitting        entity, from among the PDCP PDUs stored in the reordering        buffer, to a next processing block.    -   The PDCP receiving entity of the UE performs the reordering        operation until no stored PDCP PDU remains in the reordering        buffer.

In step 605 of FIG. 6, the UE receives a handover command from thesource ENB. In step 610, the RLC receiving entity of the UE assemblesthe qualified RLC PDUs into PDCP PDUs and delivers the assembled PDCPPDUs together with a special indication requiring reordering of the PDCPPDUs to the PDCP receiving entity.

After performing the handover to the target cell, the UE transmits ahandover complete message to the target ENB in step 615. Then, the UEremoves the existing RLC receiving entity and constructs a new RLCreceiving entity for connection with the target cell. Then, in step 620,the newly constructed RLC receiving entity performs an “in-sequencedeliver” operation for the RLC PDUs received from the target cell.

Referring to FIG. 7, when PDCP PDUs together with a special indicationrequiring reordering are delivered from the RLC receiving entity in step705, the PDCP receiving entity obtains sequence numbers of correctlyreceived PDCP PDUs and missing PDCP PDUs by checking the sequencenumbers of the delivered PDCP PDUs, and reports the obtained sequencenumbers to the RRC entity of the UE for use in reporting the PDCPreception status of the UE in step 707. The RRC entity inserts the PDCPreception status of the UE in the handover complete message transmittedto the target ENB.

In step 710, the PDCP receiving entity checks if it is necessary toreorder the PDCP PDUs delivered from the RLC receiving entity. Based ona result of the checking, the PDCP receiving entity proceeds to step 720when the reordering is necessary, and proceeds to step 715 when thereordering is unnecessary. When the reordering is necessary, it impliesthat there is at least one missing PDCP PDU.

In step 715, the PDCP receiving entity delivers the received PDCP PDUsto a next processing block. Then, in step 740, the PDCP receiving entityperforms normal operations for the PDCP PDUs received thereafter. Inother words, the PDCP receiving entity immediately delivers the PDCPPDUs from the RLC receiving entity to the next processing block.

In step 720, the PDCP receiving entity delivers the reordered PDCP PDUs,that is, PDCP PDUs having sequence numbers lower than a sequence numberof a first missing PDCP PDU, to a next processing block, and stores theremaining PDCP PDUs requiring reordering in the reordering buffer. Then,in step 725, the PDCP receiving entity waits until a PDCP PDU isdelivered from the RLC receiving entity newly constructed for the targetcell. When a PDCP PDU is delivered from the RLC receiving entity newlyconstructed for the target cell, the PDCP receiving entity proceeds tostep 730, in which the PDCP receiving entity assumes all PDCP PDUs whichhave sequence numbers higher than a sequence number of the deliveredPDCP PDU, up to the first missing PDCP PDU, as in-sequence PDCP PDUs,and outputs the assumed in-sequence PDCP PDUs to a next processingblock.

Thereafter, in step 735, the PDCP receiving entity checks ifout-of-sequence PDCP PDUs remains in the reordering buffer. Then, whenout-of-sequence PDCP PDUs remains in the reordering buffer, the PDCPreceiving entity proceeds to step 725, in which the PDCP receivingentity keeps on performing the reordering operation. Whenout-of-sequence PDCP PDUs don't remain in the reordering buffer, thePDCP receiving entity proceeds to step 740 in which the PDCP receivingentity performs the normal operations.

The first embodiment of the present invention corresponds to a casewhere a target ENB reorders PDCP PDUs received from a source ENB andPDCP PDUs received from an anchor node and then transmits the reorderedPDCP PDUs. A second embodiment of the present invention proposesoperations of PDCP and RLC when a target ENB transmits PDCP PDUs withoutreordering them.

It is preferred that the PDCP PDUs, which a target ENB receives from asource ENB, have sequence numbers always lower than those of the PDCPPDUs received by an anchor node, and that the target ENB transmits thePDCP PDUs received from the source ENB first of all. However, since thePDCP PDUs from the source ENB are delivered to the target ENB afterpassing through the anchor node, there is a possibility that they mayarrive at the target ENB later than the PDCP PDUs directly delivered tothe target ENB from the anchor node. For the sake of transmissionefficiency, it is not preferred that the target ENB stops downlinktransmission until the PDCP PDUs from the source ENB arrive.

Therefore, the target ENB first transmits a first-received PDCP PDU tothe UE from among the PDCP PDUs received from the source ENB or theanchor node. Then, it is highly probable that the UE may receive thePDCP PDUs in reverse order. Further, there is no possibility thatanother PDCP PDU having a sequence number lower than that of the PDCPPDU delivered from the source ENB may be received. However, up to thetime when there is no more PDCP PDU delivered from the source ENB, thereis still a possibility that a PDCP PDU having a sequence number lowerthan that of the PDCP PDU delivered from the anchor node may bereceived. According to the second embodiment of the present invention,the target ENB first transmits a first-arrived PDCP PDU to the UE fromamong the PDCP PDUs delivered from the source ENB or the anchor node.Further, when the target ENB transmits the PDCP PDU delivered from thesource ENB, the target ENB transmits, together with the PDCP PDU, anindication that “since the PDCP PDU is a PDCP PDU delivered from thesource ENB, there is no possibility that the PDCP receiving entity mayreceive another PDCP PDU having a sequence number lower than that of thePDCP PDU, and thus the PDCP receiving entity should deliver, uponreceiving the PDCP PDU, PDCP PDUs up to a first missing PDCP PDU havinga sequence number higher than that of the PDCP PDU to a next processingblock.” Hereinafter, for convenience of description, this indicationwill be referred to as indication 1. Briefly, indication 1 is anindication for commanding the PDCP receiving entity to apply thereordering operation according to the first embodiment of the presentinvention. Indication 1 can be delivered as control information of anRLC PDU containing at least a part of the PDCP PDU.

Referring to FIG. 8, an RLC transmitting entity of a target ENBreconstructs PDCP PDU [n] 805 delivered from a source ENB into RLC PDU[m] 810 and RLC PDU [m+1] 815, and then transmits reconstructed RLC PDU[m] 810 and RLC PDU [m+1] 815. At this time, the RLC transmitting entityof the target ENB attaches “control information 820 commanding deliveryof indication 1 together with the current RLC PDU when the PDCP PDUreconstructed into the current RLC PDU is delivered to a PDCP receivingentity” to the last RLC PDU reconstructed from PDCP PDU [n] 805, thatis, to RLC PDU [m+1] 815. Hereinafter, for convenience of description,this control information 820 is referred to as “RLC control information1.”

A PDCP receiving entity of the UE applies the reordering operationproposed by the first embodiment of the present invention to the PDCPPDU delivered together with indication 1. That is, there is nopossibility that a PDCP PDU having a sequence number lower than that ofthe PDCP PDU delivered together with indication 1 may be furtherreceived. The PDCP receiving entity ignores the possible existence of amissing PDCP PDU having a sequence number lower than that of the PDCPPDU delivered together with indication 1 and delivers all PDCP PDUshaving sequence numbers lower than a sequence number of a first missingPDCP PDU, which is higher than a sequence number of the PDCP PDUdelivered together with indication 1, to a next processing block.

When the PDCP receiving entity has received the last PDCP PDU deliveredfrom the source ENB to the target ENB, there is no possibility that aPDCP PDU having a sequence number lower than that of the last PDCP PDUmay be further received, and a further reordering operation ismeaningless. For example, even when the PDCP PDUs delivered from ananchor node are stored in a buffer of the PDCP receiving entity due to amissing PDCP PDU, there is no possibility that the missing PDCP PDU maybe received after the last PDCP PDU is received.

Therefore, according to the second embodiment of the present invention,when the target ENB transmits the last PDCP PDU received from the sourceENB to the UE, the target ENB transmits an indication 2 together withthe last PDCP PDU. Indication 2 instructs the PDCP receiving entity ofthe UE to deliver all out-of-sequence PDCP PDUs to a next processingblock. Upon receiving indication 2 together with the PDCP PDU, the PDCPreceiving entity of the UE delivers all out-of-sequence PDCP PDUs storedin the buffer to a next processing block and then performs normaloperations. Indication 2 also may be delivered as RLC controlinformation of the last RLC PDU containing at least a part of the PDCPPDU.

Referring to FIG. 8, the RLC transmitting entity reconstructs PDCP PDU825 delivered from the source ENB into RLC PDU [k] 835 and RLC PDU [k+1]840 and then transmits reconstructed RLC PDU [k] 835 and RLC PDU [k+1]840. At this time, the RLC transmitting entity attaches “controlinformation 845 commanding delivery of indication 2 together with thecurrent RLC PDU when the PDCP PDU reconstructed into the current RLC PDUis delivered to a PDCP receiving entity” to the last RLC PDUreconstructed from PDCP PDU 825, that is, to RLC PDU [k+1] 840.Hereinafter, for convenience of description, control information 845 isreferred to as “RLC control information 2.”

Referring to FIG. 9, before receiving the handover command from sourceENB 910, UE 905 receives RLC PDUs corresponding to PDCP PDU [1], PDCPPDU [2], PDCP PDU [4], and PDCP PDU [6] from the source ENB 910 in step920. From among the RLC PDUs, because RLC PDUs corresponding to PDCP PDU[1] and PDCP PDU [2] are already in sequence, they are assembled intoPDCP PDU [1] and PDCP PDU [2], which are then delivered to the PDCPreceiving entity first.

When UE 905 receives the handover command from source ENB 910 in step925, UE 905 assembles all assemblable RLC PDUs from among the RLC PDUsremaining in the RLC reception buffer into PDCP PDUs, and then deliversthe assembled PDCP PDUs to the PDCP receiving entity in step 930. Atthis time, PDCP PDU [4] and PDCP PDU [6] are assumed to have beencorrectly received and are delivered to the PDCP receiving entity. Also,the RLC receiving entity of UE 905 delivers to the PDCP receivingentity, together with the correctly received PDCP PDUs, a specialindication that the out-of-sequence PDCP PDUs requiresequence-reordering. From the special indication, the PDCP receivingentity recognizes that PDCP PDU [3] and PDCP PDU [5] are missing, andtemporarily stores PDCP PDU [4] and PDCP PDU [6], which areout-of-sequence PDCP PDUs, in a reordering buffer instead of deliveringthem to a next processing block in step 935.

After performing the handover to target ENB 915, UE 905 transmits ahandover complete message to target ENB 915 in step 940. The handovercomplete message contains information on a PDCP PDU reception status,that is, sequence numbers of missing PDCP PDUs and sequence numbers ofreceived PDCP PDUs. In the illustrated example, the handover completemessage contains PDCP PDU reception status information that PDCP PDUs upto PDCP PDU [6] have been received while PDCP PDU [3] and PDCP PDU [5]are missing.

After transmitting the handover complete message, UE 905 establishes anew RLC entity to be used in target ENB 915 in step 945, and waits forthe arrival of RLC PDUs to the new RLC receiving entity from target ENB915.

Meanwhile, target ENB 915 receives PDCP PDUs from source ENB 910 and theanchor node. Here, the description is based on a case where the PDCPPDUs from the anchor node arrive first. That is, PDCP PDUs from PDCP PDU[9] start to be received from the anchor node in step 929, and PDCP PDU[3] to PDCP PDU [8] are received from source ENB 910 in step 927. TheRLC transmitting entity newly constructed in target ENB 915 stores thereceived PDCP PDUs in a transmission buffer in the order in which theywere received. Specifically, they are stored in the transmission bufferin an order of PDCP PDU [9], PDCP PDU [10], PDCP PDU [11], PDCP PDU [3],PDCP PDU [5], PDCP PDU [6], PDCP PDU [7], PDCP PDU [8], PDCP PDU [12],and so on.

Then, upon receiving the handover complete message from UE 905, targetENB 915 determines the PDCP PDUs to be transmitted by referring to thePDCP PDU reception status information contained in the handover completemessage, and then transmits the determined PDCP PDUs by referring to thesequence numbers of the determined PDCP PDUs in step 950. In theillustrated example, from among PDCP PDU [3]˜PDCP PDU [8] received fromsource ENB 910, only PDCP PDU [3], PDCP PDU [5], PDCP PDU [7], and PDCPPDU [8] are transmitted to UE 905. Also, the PDCP PDUs are transmittedin the order in which they are stored in the transmission buffer, thatis, in an order of PDCP PDU [9], PDCP PDU [10], PDCP PDU [11], PDCP PDU[3], PDCP PDU [5], PDCP PDU [6], PDCP PDU [7], PDCP PDU [8], and PDCPPDU [12]. Since a sequence number of the RLC transmitting entityconstructed in target ENB 915 is initialized to 0, target ENB 915 grantsa sequence number of 0 to the first RLC PDU corresponding to PDCP PDU[3].

UE 905 performs the “in-sequence delivery” operation according to an RLCreceiving operation by the new RLC receiving entity in step 955,assembles the in-sequence RLC PDUs into PDCP PDUs, and then delivers theassembled PDCP PDUs to the receiving entity in step 960. At this time,the PDCP PDU constructed by assembling the RLC PDUs containing RLCcontrol information 1 is delivered together with indication 1 to thePDCP receiving entity, and the PDCP PDU constructed by assembling theRLC PDUs containing RLC control information 2 is delivered together withindication 2 to the PDCP receiving entity.

Upon receiving the PDCP PDU from the RLC receiving entity newlyconstructed in the target cell, the PDCP receiving entity checks if thereceived PDCP PDU is accompanied by indication 1 or indication 2. Whenthe received PDCP PDU is accompanied by indication 1, the PDCP receivingentity assumes that the PDCP PDUs are in sequence up to the receivedPDCP PDU, and delivers PDCP PDUs up to the first missing PDCP PDU havinga sequence number higher than that of the received PDCP PDU to a nextprocessing block in step 965. In contrast, when the received PDCP PDU isaccompanied by indication 2, the PDCP receiving entity assumes that allthe PDCP PDUs stored in the buffer including the received PDCP PDU arein sequence, and delivers them to a next processing block in step 970.

In step 1005 of FIG. 10, the UE receives a handover command from thesource ENB. In step 1010, the RLC receiving entity of the UE assemblesthe qualified RLC PDUs into PDCP PDUs and delivers the assembled PDCPPDUs together with a special indication requiring reordering of the PDCPPDUs to the PDCP receiving entity.

After performing the handover to the target cell, the UE transmits ahandover complete message to the target ENB in step 1015. Then, the UEremoves the existing RLC receiving entity and constructs a new RLCreceiving entity for connection with the target cell. Then, in step1020, the newly constructed RLC receiving entity performs an“in-sequence deliver” operation for the RLC PDUs received from thetarget cell. In step 1025, the newly constructed RLC receiving entitydelivers the in-sequence PDCP PDUs to the PDCP receiving entity. At thistime, the PDCP PDU constructed by assembling the RLC PDUs containing RLCcontrol information 1 is delivered together with indication 1 to thePDCP receiving entity, and the PDCP PDU constructed by assembling theRLC PDUs containing RLC control information 2 is delivered together withindication 2 to the PDCP receiving entity.

Referring to FIG. 11, when PDCP PDUs together with a special indicationrequiring reordering are delivered from the RLC receiving entity in step1105, the PDCP receiving entity obtains sequence numbers of correctlyreceived PDCP PDUs and missing PDCP PDUs by checking the sequencenumbers of the delivered PDCP PDUs, and reports the obtained sequencenumbers to the RRC entity of the UE for use in reporting the PDCPreception status of the UE in step 1107. The RRC entity inserts the PDCPreception status of the UE in the handover complete message transmittedto the target ENB.

In step 1110, the PDCP receiving entity checks if it is necessary toreorder the PDCP PDUs delivered from the RLC receiving entity. Based onthe result, the PDCP receiving entity proceeds to step 1120 when thereordering is necessary, or proceeds to step 1115 when the reordering isunnecessary. When the reordering is necessary, it implies that there isat least one missing PDCP PDU.

In step 1115, the PDCP receiving entity delivers the received PDCP PDUsto a next processing block. Then, in step 1140, the PDCP receivingentity performs normal operations for the PDCP PDUs received thereafter.In other words, the PDCP receiving entity immediately delivers the PDCPPDUs to the next processing block as soon as receiving the PDCP PDUsfrom the RLC receiving entity.

In step 1120, the PDCP receiving entity delivers the reordered PDCPPDUs, that is, PDCP PDUs having sequence numbers lower than the sequencenumber of the first missing PDCP PDU, to the next processing block, andstores the remaining PDCP PDUs requiring the reordering in thereordering buffer. Then, in step 1125, the PDCP receiving entity waitsuntil a PDCP PDU is delivered from the RLC receiving entity newlyconstructed for the target cell. When a PDCP PDU is delivered from theRLC receiving entity newly constructed for the target cell, the PDCPreceiving entity proceeds to step 1127 in which the PDCP receivingentity determines if the PDCP PDU is accompanied by indication 2. ThePDCP receiving entity proceeds to step 1128 when the PDCP PDU isaccompanied by indication 2, and proceeds to step 1129 when the PDCP PDUis not accompanied by indication 2.

Since indication 2 indicates that there is no possibility that anotherPDCP PDU having a sequence number lower than that of the delivered PDCPPDU may be further received and there is no possibility that any of thePDCP PDUs currently stored in the reordering buffer may be reordered tobecome in-sequence, the PDCP receiving entity delivers all the PDCP PDUsstored in the reordering buffer to a next processing block and thenproceeds to step 1140 in which the PDCP receiving entity performs normaloperations.

When the PDCP PDU is not accompanied by indication 2, the PDCP receivingentity proceeds to step 1129 in which the PDCP receiving entitydetermines if the PDCP PDU is accompanied by indication 1. Sinceindication 1 indicates that there is no possibility that another PDCPPDU having a sequence number lower than that of the delivered PDCP PDUmay be further received, the PDCP receiving entity proceeds to step 1130in which the PDCP receiving entity assumes all PDCP PDUs, which havesequence numbers higher than a sequence number of the delivered PDCPPDU, up to the first missing PDCP PDU, as in-sequence PDCP PDUs, andoutputs the assumed in-sequence PDCP PDUs to a next processing block.

Thereafter, in step 1135, the PDCP receiving entity checks ifout-of-sequence PDCP PDUs remains in the reordering buffer. Then, whenout-of-sequence PDCP PDUs remains in the reordering buffer, the PDCPreceiving entity proceeds to step 1125 in which the PDCP receivingentity keeps on performing the reordering operation. Whenout-of-sequence PDCP PDUs do not remain in the reordering buffer, thePDCP receiving entity proceeds to step 1140 in which the PDCP receivingentity performs the normal operations.

Meanwhile, when the determination in step 1129 concludes that the PDCPPDU is not accompanied by indication 1, the PDCP receiving entityproceeds to step 1133 in which the PDCP receiving entity stores the PDCPPDU according to its sequence number in the reordering buffer. Then, thePDCP receiving entity proceeds to step 1125 in which the PDCP receivingentity performs the normal operations.

Referring to FIG. 12, an RLC transmitting entity 1270 of a target ENBincludes a transmission buffer 1215, a segmentation/header adding unit1220, a retransmission buffer 1225, and an RLC control unit 1230. Thetransmission buffer 1215 stores PDCP PDUs 1205 delivered from a sourceENB and PDCP PDUs 1210 delivered from an anchor node. According to thefirst embodiment of the present invention, transmission buffer 1215stores the delivered PDCP PDUs after ordering them according to theirsequence numbers. According to the second embodiment of the presentinvention, transmission buffer 1215 stores the delivered PDCP PDUsaccording to the order in which the PDCP PDUs were delivered.

Transmission buffer 1215 delivers the PDCP PDUs appointed by RLC controlunit 1230 from among the stored PDCP PDUs to segmentation/header addingunit 1220 and removes the delivered PDCP PDUs.

Segmentation/header adding unit 1220 segments or concatenates the PDCPPDUs delivered from transmission buffer 1215 into PDCP PDUs each havingthe proper size, and then inserts an RLC header including an RLCsequence number, etc. into the PDCP PDUs, thereby reconstructing atleast one RLC PDU. At this time, if RLC control information has beentransmitted from RLC control unit 1230, segmentation/header adding unit1220 inserts the RLC control information into a predetermined locationof the reconstructed RLC PDU. The RLC PDU is delivered to retransmissionbuffer 1225 and a lower layer. The RLC PDU delivered to the lower layeris delivered to an RLC receiving entity 1275 of the UE according to apredetermined sequence, and the RLC PDU delivered to retransmissionbuffer 1225 is stored until an ACK signal from RLC receiving entity 1275arrives.

RLC control unit 1230 controls transmission and retransmission of RLCPDUs or PDCP PDUs stored in transmission buffer 1215 and retransmissionbuffer 1225. Specifically, RLC control unit 1230 receives sequencenumbers of PDCP PDUs, which the UE correctly received from the sourceENB, from an RRC entity (not shown), and removes the PDCP PDUs fromtransmission buffer 1215. Further, when there is RLC control informationto be transmitted to the UE, RLC control unit 1230 delivers the RLCcontrol information to segmentation/header adding unit 1220, so that theRLC control information can be piggybacked on the transmitted RLC PDU.According to the second embodiment of the present invention, when thePDCP PDU delivered from transmission buffer 1215 is the PDCP PDUdelivered from the source ENB, RLC control unit 1230 controlssegmentation/header adding unit 1220 so that RLC control information of1 is attached to a corresponding RLC PDU. At the time of transmitting alast PDCP PDU delivered from the source ENB, RLC control unit 1230controls segmentation/header adding unit 1220 so that RLC controlinformation of 2 is attached to a corresponding RLC PDU.

RLC receiving entity 1275 includes a reception buffer 1235, an RLCcontrol unit 1245, and a reassembly unit 1240. Reception buffer 1235stores the RLC PDUs received from the ENB according to RLC sequencenumbers. From among the in-sequence RLC PDUs, RLC PDUs fit to beassembled as PDCP PDUs are delivered from reception buffer 1235 toreassembly unit 1240. At this time, if the RLC PDU includes controlinformation, the control information is delivered to RLC control unit1245.

Reassembly unit 1240 assembles the RLC PDUs delivered from receptionbuffer 1235 into PDCP PDUs and then delivers the PDCP PDUs to a PDCPreceiving entity 1280.

When the UE receives a handover command, RLC control unit 1245 controlsreception buffer 1235 that reception buffer 1235 delivers all RLC PDUsfit to be assembled as PDCP PDUs from among the RLC PDUs stored inreception buffer 1235 to reassembly unit 1240. Further, RLC control unit1245 delivers a special indication to a reordering control unit 1265 ofPDCP receiving entity 1280 to request a reordering buffer 1250 to storethe PDCP PDUs delivered at a corresponding time point (i.e. a time pointat which the special indication is delivered) until they become insequence. Meanwhile, when the handover of the UE is completed, an RRCentity 1260 collects PDCP PDU reception status information fromreordering control unit 1265 and inserts the collected PDCP PDUreception status information into the handover complete messagetransmitted to the target ENB.

According to the second embodiment of the present invention, if RLCreceiving entity 1275 has received RLC control information 1 togetherwith the RLC PDU from the target ENB, RLC control unit 1245 deliversindication 2 to reordering control unit 1265 of PDCP receiving entity1280 together with the PDCP PDU assembled from the RLC PDUs containingthe RLC control information 2.

PDCP receiving entity 1280 includes a reordering buffer 1250, areordering control unit 1265, and a next processing block 1255.Processing block 1255 may include, for example, a deciphering unit and aheader decompression unit in order to process a higher layer protocol inrelation to the service being provided to the UE.

Normally, reordering buffer 1250 directly delivers a PDCP PDU deliveredfrom RLC receiving entity 1275 to next processing block 1255. However,when it receives a reordering command from reordering control unit 1265,reordering buffer 1250 stores the PDCP PDUs required to be reordered.

According to the first embodiment of the present invention, whenreordering buffer 1250 receives a PDCP PDU from the newly constructedRLC receiving entity 1275 while the PDCP PDUs required to be reorderedare stored in reordering buffer 1250, reordering buffer 1250 assumesPDCP PDUs up to the first missing PDCP PDU having a sequence numberhigher than that of the received PDCP PDU to be in sequence and deliversthe PDCP PDUs to next processing block 1255. Further, when there is nostored PDCP PDU, reordering buffer 1250 performs normal operations againand directly delivers received PDCP PDUs to next processing block 1255.

According to the second embodiment of the present invention, whenreordering buffer 1250 receives a PDCP PDU together with the indication1, reordering buffer 1250 assumes PDCP PDUs up to the first missing PDCPPDU having a sequence number higher than that of the received PDCP PDUto be in sequence and directly delivers the PDCP PDUs to next processingblock 1255. Further, when reordering buffer 1250 receives a PDCP PDUtogether with the indication 2, reordering buffer 1250 assumes allstored PDCP PDUs to be in sequence and delivers all the PDCP PDUs tonext processing block 1255.

Next processing block 1255 reconstructs an IP packet by deciphering thePDCP PDUs delivered from reordering buffer 1250 and restoring theheader, and then delivers the IP packet to a higher layer (e.g. IPlayer).

Now, effects of the present invention, which has the construction andoperation as described above, will be briefly described.

According to the present invention, in handover re-establishing an ARQentity in a mobile communication system, a PDCP entity, which is ahigher layer of the ARQ entity, performs a reordering operation, so thata target cell can selectively retransmit packets, which a UE has notreceived in a source cell, to the UE. Therefore, the present inventioncan improve efficiency of the communication.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A data communication method in a wirelesscommunication system including a radio link control (RLC) entity and apacket data convergence protocol (PDCP) entity, the method comprising:receiving a message associated with a handover; assembling at least onePDCP packet from at least one RLC packet; delivering the assembled atleast one PDCP packet from the RLC entity to the PDCP entity; storingthe delivered at least one PDCP packet in a PDCP buffer; generating PDCPstatus information and transmitting the PDCP status information;re-establishing the RLC entity; receiving, by the PDCP entity from theRLC entity, a new PDCP packet having a sequence number (SN); anddelivering, by the PDCP entity to a higher layer, the at least one PDCPpacket with an associated SN lower than the SN associated with thereceived new PDCP packet and the at least one PDCP packet with aconsecutively associated SN starting from the SN associated with thereceived new PDCP packet.
 2. The method of claim 1, wherein the PDCPstatus information indicates a reception status of the at least one PDCPpacket.
 3. The method of claim 1, wherein the PDCP status information istransmitted to a target eNB.
 4. The method of claim 1, wherein there-established RLC entity is used in a target cell.
 5. The method ofclaim 4, further comprising: checking at least one PDCP packet with theassociated SN lower than the SN associated with the received new PDCPpacket, and at least one PDCP packet with the consecutively associatedSN starting from the SN associated with the received new PDCP packet. 6.The method of claim 1, wherein the at least one PDCP packet with theassociated SN lower than the SN associated with the received new PDCPpacket and the at least one PDCP packet with the consecutivelyassociated SN starting from the SN associated with the received new PDCPpacket are correctly received before the re-establishing the RLC entity.7. The method of claim 1, wherein the PDCP status information comprisesat least one SN associated with at least one missing PDCP packet.
 8. Themethod of claim 1, wherein the PDCP status information comprises atleast one SN associated with at least one correctly received PDCPpacket.
 9. A user equipment (UE) apparatus for receiving a messageassociated with a handover in a wireless communication system, the UEapparatus comprising: a radio link control (RLC) entity configured toassemble at least one PDCP packet from at least one RLC packet, deliverthe assembled at least one PDCP packet to a packet data convergenceprotocol (PDCP) entity, and be re-established; and the PDCP entityconfigured to store the delivered at least one PDCP packet in a PDCPbuffer, generate PDCP status information, transmit the PDCP statusinformation, receive from the RLC entity a new PDCP packet having asequence number (SN), and deliver to a higher layer the at least onePDCP packet with an associated SN lower than the SN associated with thereceived new PDCP packet and the at least one PDCP packet with aconsecutively associated SN starting from the SN associated with thereceived new PDCP packet.
 10. The UE apparatus of claim 9, wherein thePDCP status information indicates a reception status of the at least onePDCP packet.
 11. The UE apparatus of claim 9, wherein the PDCP entitytransmits the PDCP status information to a target eNB.
 12. The UEapparatus of claim 9, wherein the re-established RLC entity is used in atarget cell.
 13. The UE apparatus of claim 12, wherein the PDCP entityis further configured to: check at least one PDCP packet with theassociated SN lower than the SN associated with the received new PDCPpacket, and at least one PDCP packet with the consecutively associatedSN starting from the SN associated with the received new PDCP packet.14. The UE apparatus of claim 9, wherein the at least one PDCP packetwith the associated SN lower than the SN associated with the receivednew PDCP packet and the at least one PDCP packet with the consecutivelyassociated SN starting from the SN associated with the received new PDCPpacket are correctly received before the RLC entity is re-established.15. The UE apparatus of claim 9, wherein the PDCP status informationcomprises at least one SN associated with at least one missing PDCPpacket.
 16. The UE apparatus of claim 9, wherein the PDCP statusinformation comprises at least one SN associated with at least onecorrectly received PDCP packet.